1. Field of the Invention
The present invention relates to a focus controlling apparatus and method for focusing a light beam and positioning the focused light beam onto a signal recording layer of a recording media, and an optical disc drive for recording and/or reproducing signal with respect to an optical disc by positioning a focused light beam onto the signal recording layer of the optical disc.
2. Description of Related Art
In an optical disc drive adapted to recording and/or reproducing signal by positioning a focused light beam onto a signal recording layer of an optical disc, a focus servo is employed to control the focus of an optical system in the optical disc drive by converging a reflected light from the optical disc by an optical system and checking whether the light is accurately focused or not for. Typically, the astigmatism or knife-edge method is used for detection of such focused status.
The astigmatism method is such that an optical element such as a cylindrical lens which will cause an astigmatism is used to generate an astigmatism in a light beam to detect a focus error on the basis of the fact that when the signal recording layer of the optical disc is off the focal surface, the reflected light beam will have the section thereof varied in shape from circular to elliptic. In the knife-edge method, a focus error is detected by disposing a knife edge over the width of an optical path along which the reflected and focused light from the optical disc propagates and detecting when the light spot image leaves a photodetector element as the signal recording layer of the optical is off the focal surface.
In an optical system using an objective lens having a numerical aperture of 0.6 or more, however, the depth of focus of the objective lens is so small that the focus error allowance for the focus servo (allowable defocusing) is extremely small. The depth of focus of the objective lens is expressed as (xcex/NA)2. Therefore, the depth of focus of the optical system 100 in which the laser light wavelength xcex is 640 nm and numerical aperture (NA) of the objective lens is 0.8, for example, is as remarkably small as about ⅕ of that of an optical system in which xcex=780 nm and NA=0.45) for compact discs (CD). Thus, it is necessary to always keep the working focus error within an allowable range by optimizing a residual focus error (offset) being a steady-state deviation between a focused light beam positioned by the focus servo and the signal recording layer of a signal recording media.
For a higher density of signal recording, some optical discs are adapted to record signal on both the grooves and lands formed thereon, or other discs are adapted to use two or more recording layers for recording and/reproducing signal. For signal recording and/or recording signal with respect to such optical discs, a more accurate focus control is required.
One of the factors on which the focus servo offset depends is an assembling inaccuracy. To compensate such an assembling error or inaccuracy, it has been a countermeasure used in the prior art that the offset is roughly adjusted during assembling of an optical pick-up and the residual focus error is controlled to less than its allowance by the focus servo. Thus, the conventional optical pick-up using an objective lens having a high NA has to employ a heavy-duty biaxial actuator having a sufficient gain and band.
It should be assumed here that the oscillation of an optical disc being spun is xc2x140 xcexcm and its fundamental frequency is 60 Hz because the oscillation is synchronous with the optical disc spinning. Then, to limit the offset to a value sufficiently lower than its allowance, the servo gain should be 40 to 50 dB at the above fundamental frequency.
For a focus control using such a heavy-duty biaxial actuator, however, there arise problems such as a larger structure of the optical pick-up and larger power consumption. Also, in optical discs such as a one using both lands and grooves for signal recording and/reproduction, a one having two or more recording layers for signal recording and/or reproduction, etc., an optimum offset of the focus servo varies from one recording position to another on a same optical disc.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a focus controlling apparatus and method adapted to automatically adjust a focus servo offset using a conventional actuator for moving an objective lens of an NA as high as 0.6 or more employed in an optical system. Moreover, the present invention has another object to provide an optical disc drive in which the above-mentioned focus controlling method is applied to play a normal read-only optical disc, record and reproduce signal with respect to an optical disc using either grooves or land thereof for signal recording, and record and reproduce signal with respect to a high recording density optical disc designed to record signal at both lands and grooves thereof.
The above object can be accomplished by providing a focus controlling apparatus and method, according to the present invention, in which an offset between a focused light beam positioned by a light beam focusing means of an NA over 0.6 and the signal recording layer of an recording media can be adjusted depending upon a signal read from the recording media to automatically optimize a focus servo offset by a conventional objective lens actuator.
The above object can also be accomplished by providing an optical disc drive designed, according to the present invention, to record and/or reproduce signal with respect to an optical disc using both grooves and lands thereof for signal recording and an optical disc of a high recording density having a plurality of recording layers.